I'd like to tack on a couple tips:
Tolerance & stack-up, GD&T.
Unless your machine is capable of handling tenths (.0001") of accuracy, plan on a couple thousandths (.002-.004"); even .005" can be easily massaged (lapped) with deburring / sanding (even aluminum, and to a lesser extent, steel).
When/if planning for mating surfaces, plan for tolerances into your design... MANY "fresh-out" designers will calculate a 1/8" dowel-pin as exactly .1250", while not realizing a 1/8" dowel-pin is going to be over-sized by several tenths, typically .00025-.0003", as they're typically intended to be used as a press-fit with a slight interference.
If dimensioning parts from one feature to another, each will assume a standard tolerance:
|---|----o----|-----| = measuring from feature to feature will incur 4x the standard tolerance, opposed to one measurement from edge to edge (IE, measuring feature A to B, then B to C, and C to D ... instead of providing measurement A to D, means a possible difference in absolute dimensions.)
More on any of this, if it's necessary.
Know your proper speed & feed, and how to calculate the proper chip-load per material, tool/cutter, and machine-performance. Additionally, higher feed (how fast the machine moves) with slow speed (spindle/tool RPM) can cause damage to the machine, break the tool, and/or scrap your part. Lastly, the surface finish can be improved with proper numbers.
Where speed is needed:
The smaller the diameter of your tool becomes, the greater the RPM is needed (and lower feed). If using a .005" end-mill for high detail, you'll need a 20k+RPM spindle, and VERY slow feed (movement). Unfortunately, I'd expect that most hobby-machines like a Sherline, Taig, or other bench-top machine, should be kept to 1/16" diameter tools at the smallest, because a simple bump to the machine can destroy that tool (and again, your part.) Heavier machines (~1000lb+) are much less susceptible to this.
Know your machine:
Closed-loop servo-systems with glass-scales will tell you the position of the tool (.001" to .0002" and better, depending on machine-quality) vs that of an open-loop or Stepper-motor system that will only tell you where the control THINKS it should be. Programming moves based on the possibility of error will reduce quality. Keep in mind, that tool run-out (how "true" the concentricity is of the spindle, collet (tool-holder), bearings and machine-movement (drives)) directly effect the quality and tolerance of the cut.
ALWAYS do your due-diligence!
IF you have doubts about your program, verify your numbers or run a simulation if possible. Perform a dry-run a safe distance above the part to ensure the tool-path looks right. Verify Counter-Clockwise vs Clock-Wise arcs - these are part-killers when run with an error.
A lightweight machine can snag a shirt-sleeve and draw you in instantly. You might want to touch the tool while it's spinning, but really, don't. Use brushes, air, coolant, or alcohol squirt-bottles to remove chips. DO NOT use your fingers. It might be safe, but it's the best practice to be good at. Even great machinists can lose a part of a finger.
IF running a program you're unsure about, it's OK to grit your teeth - but just keep your finger on the HOLD and/or emergency shut-off. It's better to shut down the machine and start over if you're unsure, than to 'ride it out'.
Also, glasses with side-shields! The proper method of removing metal will remove the HEAT generated by the cut, in the chip; and they're also sharp. Those chips DO fly out, and can fly far. The LAST place you want it (at LEAST the second to last place), is in your eye. You'll get them in your hair (assuming you have some), in your nose, and in your mouth (unless closed). Just expect it.